trace_processor: add support for creating protos from SQL

Implements go/perfetto-metrics-sql-proto for trace processor.

Context: go/perfetto-metrics
Bug: 129747127
Change-Id: Id8516f7478850009baee117a988b9d9598a15e49
diff --git a/src/trace_processor/metrics/BUILD.gn b/src/trace_processor/metrics/BUILD.gn
index 0ca3908..b05d0cd 100644
--- a/src/trace_processor/metrics/BUILD.gn
+++ b/src/trace_processor/metrics/BUILD.gn
@@ -40,6 +40,8 @@
 
 source_set("lib") {
   sources = [
+    "descriptors.cc",
+    "descriptors.h",
     "metrics.cc",
     "metrics.descriptor.h",
     "metrics.h",
@@ -49,6 +51,7 @@
     "../../../buildtools:sqlite",
     "../../../gn:default_deps",
     "../../../include/perfetto/trace_processor",
+    "../../../protos/perfetto/common:zero",
     "../../../protos/perfetto/metrics:zero",
     "../../../protos/perfetto/metrics/android:zero",
     "../../protozero:protozero",